The use of articulating block matrices for soil erosion prevention is known in the art. Typically, such systems involve the grading of an embankment or shoreline to a predetermined slope, the installation of a highly water permeable geosynthetic fabric over the soil substrate, and then the placement over the fabric of a matrix of blocks. A typical matrix of blocks is comprised of precast concrete blocks. Such blocks may be tied together into mats with cables usually comprised of high strength polyester or galvanized steel. Alternatively, the formation of the matrix may rely soley on the interlock provided by the block's design. Cabled mats are typically assembled off-site at a block precasting facility. After the blocks are cast, cables are strung through tunnels in the blocks, typically producing mats that are approximately 8 feet wide and 40 feet long. Mats of this size have proven convenient for handling and transporting to the job site. The assembled mats are lifted onto a truck or barge for transportation to the job site using a crane or large forklift truck equipped with a spreader bar assembly which suspends the mats in a generally horizontal orientation. At the installation site, the mats are placed side by side by a crane using a spreader bar assembly. The cables of adjacent mats are bonded together so that the finished installation comprises a continuous matrix of concrete blocks. Alternatively, the blocks may be placed individually and, if desired, cabled together after they are laid into a matrix.
The resulting surface may have openings between the blocks and/or in the blocks that may be backfilled with soil and seeded to produce vegetation. The presence of vegetation produces an aesthetically appealing shoreline and also provides greater resistance to erosion.
A revetment system constructed in this manner relies on the combination of the permeable fabric and the articulating concrete block surface to overcome the erosive effects of flowing water or waves to hold in place the underlying soil. Such systems have been widely used, and there are numerous examples of revetment systems that operate in the general fashion described above, including those described in U.S. Pat. No. 4,227,829 (Landry), U.S. Pat. No. 4,370,075 (Scales) and systems such as that marketed by Petratech, Inc. under the tradename PETRAFLEX.TM. Revetment System and that marketed by Nicolon Corporation under the trade designation ARMORLOC.
The revetment system described in the Landry patent is referred to as a "dual cable system" because one set of cables passes through the entire transverse dimension of the matrix and another set passes through the entire longitudinal dimension of the matrix. The blocks have angular tapered sides such that the top surface of the block has less surface area than the bottom surface, to facilitate articulation of the matrix over non-planar surfaces and bowing of the matrix when it is suspended from a spreader bar assembly.
The revetment system described in Scales is also a matrix of blocks placed in parallel transverse rows, with cable interconnections. The blocks also have angular tapered sides to facilitate articulation. Unlike Landry, the revetment system described in Scales uses cables that travel only in the longitudinal direction and each block has two longitudinal tunnels for the cables. This system typically is referred to as a "single cable system". The blocks of Scales are of a generally rectangular shape, with recesses and protrusions in the sidewalls configured so that longitudinally adjacent blocks interlock when the blocks are placed in a "running bond" pattern in the matrix by off-setting adjacent transverse rows in the transverse direction.
In the PETRAFLEX.TM. System, the blocks are generally square, and are placed in parallel columns and rows with a dual cable system. Two tunnels, each accepting one cable, are used in the longitudinal direction, and one tunnel, accepting one cable, is oriented in the transverse direction. Unlike Landry, the block of the PETRAFLEX.TM. system has, for each pair of sidewalls, one male tab on a side opposed to one female tab on the other side to interlock adjacent blocks when placed in a matrix with parallel rows and columns of like blocks.
In the ARMORLOC system, the blocks may be generally rectangular or square and are placed in offset rows and columns. A block in this system can be held in place by interlocking with as many as four adjacent blocks.
Another important design consideration for revetment systems is their ability to allow water to flow through the surface of the concrete mats. In most settings where such systems are used, water may be present in the soil substrate underneath the layer of geotextile and the concrete block mat. Such water may be introduced through rainfall, surface flows, wave action, subsurface groundwater flows or other elements. As a result, it is highly desirable that the surface of the block matrix be permeable so that the matrix is not displaced by hydrostatic pressure or undermined by erosion caused by flows occurring in the soil substrate beneath the block matrix and geotextile. It is common practice to have open voids in the matrix consist of approximately twenty percent (20%) of the total surface of the block matrix. Such voids are located either within the blocks or in the spaces between the blocks when they are placed in the matrix. There are also instances, however, where a unit without such open voids may be desired.
While such openings are highly desirable, they do introduce an element of vulnerability to displacement of the blocks, because such voids may allow wave action or water flows to destabilize or undermine the matrix. Thus, the voids should be designed to minimize the disruptive effect of hydrodynamic forces while providing sufficient open area to allow the release of water that may accumulate beneath the surface of the matrix.
The manner in which the blocks are placed into a matrix is an important design feature of articulating block revetment systems. The art teaches the use of cables connecting the blocks and providing a block to block interlock by shaping the blocks so that they nest together when placed in a matrix. The art also includes blocks that are laid without using interconnecting cables and which rely on the block's interlock with adjacent blocks in the matrix. The dual cable systems perform well, but require additional cable over that required by the single cable systems. Systems in the art not using any cables have not performed as well as cabled systems, but may be more cost-effective for certain applications. While the use of cables is desirable for system strength and to prevent removal by vandals, blocks without cables can be hand-placed, which has advantages in certain applications. For example, for small areas, it would be advantageous to avoid the use of heavy moving equipment by simply placing the necessary blocks by hand. For larger areas where, for example, below-water installation is not necessary, a hand-placed block may be more cost effective than the placement of cabled mats.
Hand placement of the blocks, however, is an advantage if there is sufficient interlock between the blocks to hold them in place. Thus, there is a need for a block useful in a revetment system with good interlock between adjacent blocks. Such a block would provide optimal resistance to erosion and displacement due to its interlocking design. Such a block also should be able to meet the design requirements of varying site conditions, including having the necessary hydrodynamic efficiency.